1. Field of the Invention
The present invention relates to an X-ray imaging apparatus that performs radiographing using X-rays that have transmitted through an object.
2. Description of the Related Art
In recent years, in the medical fields, in place of an analog X-ray imaging apparatus using a film for conventional X-ray diagnosis, a digital X-ray imaging apparatus using a flat panel detector in which pixels composed of semiconductor devices for converting X-rays into electric signals are disposed in a two-dimensional matrix pattern is in widespread use.
In such a digital X-ray imaging apparatus, information of X-rays that have passed through an object is stored in pixels as electric charges. Thereafter, the digital X-ray imaging apparatus performs transfer operation, and reads out the information as voltage, and subsequently performs A/D conversion to acquire digital image information. However, in the electric charge transfer operation of the pixels, it is known that electric charges cannot be all discharged, and residual electric charge appears. Further, accumulation of electric charges in the pixels may alter characteristics of the semiconductor devices and thus may lower their sensitivities.
It is known that, due to these phenomena, when the digital X-ray imaging apparatus is used, a portion of radiographic information of the previous radiographic image appears as an image lag at the time of the next radiographing operation.
Since signals which are not actually present in clinical images will be output as an artifact resulting from such an image lag, it has become a problem in terms of diagnosis. In particular, the kind of image lag generated by change of characteristics of the elements caused by the latter X-ray irradiation, and by degradation of sensitivity includes a component which does not fully decay even in a minute or so, and continues to give influence on diagnosis images for a long period of time. As a result, they have caused impairing of clinical values of the digital X-ray imaging apparatus.
In the present invention, of the image lags of the X-ray imaging apparatus, the one resulting from a large time constant of the above-described decay is handled, which is simply referred to as an image lag.
In order to inhibit such an image lag, a technique for erasing the image lag by shifting to a state where power supply to the semiconductor devices is temporarily stopped each time radiography ends (see, for example, Japanese Patent Application Laid-Open No. 2002-199278) is known. Further, there is known a technique for stabilizing sensitivity and inhibiting generation of the image lag by installing a separate light source unit, and irradiating whole surface of a detector with light rays (see, for example, Japanese Patent Application Laid-Open No. 2007-181183).
However, the above-described conventional technique has a problem. That is, in the technique discussed in Japanese Patent Application Laid-Open No. 2002-199278, although the image lag inhibiting effect is high, a time that elapses before the next radiography becomes possible (hereinafter, referred to as radiography cycle time) will becomes longer. Further, in the technique discussed in Japanese Patent Application Laid-Open No. 2007-181183, sensors will become larger in size, heavier in weight, and higher in cost.